Semiconductor laser element, semiconductor etchant, and method of fabricating the semiconductor laser element

ABSTRACT

The semiconductor laser element comprises, from bottom to top, the p-Al x Ga 1−x As upper clad layer, p-Al y Ga 1−y As resistance control layer, and p-GaAs cap layer (where x&gt;y&gt;0.2). A portion of only the resistance control layer and cap layer is selectively etched. The etchant used for this etching is a mixture of organic acid and hydrogen peroxide based mixture, has such a composition such that the ratio of dissolution rate of the upper clad layer to the cap layer is between 10 and 20, and pH is between 7.4 and 7.6.

FIELD OF THE INVENTION

[0001] The present invention relates to a semiconductor laser elementhaving a current non-injection region. This invention also relates to asemiconductor etchant (“etchant”) used for selectively etching acompound semiconductor crystal, and a method of fabricating thesemiconductor laser element using the semiconductor etchant.

BACKGROUND OF THE INVENTION

[0002] GaAs-based semiconductor laser elements are widely used inexcitation light sources of optical amplifiers and the like. When theGaAs-based semiconductor laser element is used in excitation lightsources, it is necessary that its light output is high. However, whenthe light output of the semiconductor laser element is increased,following phenomenon disadvantageously occur at the laser end facet ofthe semiconductor laser element. Firstly there occurs an optical damage,secondly there occurs a corrosion of the laser end facet when the laserelement is operated over a long period. It is believed that thesephenomenon are caused because of increase of the temperature of the endfacet (resonator surface), contraction of the band gap,photo-absorption, recombination current, and a combination of one ormore of these.

[0003] When the light output of the semiconductor laser element isincreased, the optical damage and end facet corrosion become moreconspicuous because the light density at the end facet increases.Sometimes the deterioration is so high that the generation of laser issuddenly stopped. In order to overcome these problems, it is desirableto have a semiconductor laser element in which light intensity isreduced only near the end facet.

[0004] As a countermeasure, Japanese Patent Application Laid-Open No.6-188511 discloses a semiconductor laser element which has resonator endfacets with different reflectances, and a ridge mesa on an active layer.The ridge mesa is formed in the region except in a region near theresonator end facet on the low reflectance side. At least a part of theregion where the ridge mesa is formed is provided with a currentnon-injection structure.

[0005] A cross-section of the semiconductor laser element proposed inthe above-mentioned reference is shown in FIG. 9. A cross section of thesemiconductor laser element along the line A-A shown in FIG. 9 is shownin FIG. 10. This semiconductor laser element is fabricated by thefollowing method.

[0006] 1) An epi-wafer is fabricated by stacking a plurality of layer onn-GaAs substrate 1. In this epi-wafer, n-GaAs (n=1×10¹⁸ cm⁻³) bufferlayer 2 of thickness 0.5 μm, n-AlGaAs (n=1×10¹⁸ cm⁻³) lower clad layer 3of thickness 1.5 μm, n-GaAs (n=3×10¹⁷ cm⁻³) lower optical confinementlayer 4 of thickness 0.03 μm, p-In_(0.2)Ga_(0.8)As (p=3×10¹⁷ cm⁻³)active layer 5 of thickness 80 Å, p-GaAs (p=3×10¹⁷ cm⁻³) upper opticalconfinement layer 6 of thickness 0.03 μm,p-Al_(0.35)Ga_(0.65)As(p=1×10¹⁸ cm⁻³) upper clad layer 7 of thickness1.2 μm, and p-GaAs(p=4×10¹⁹ cm⁻³) cap layer 9 of thickness 0.5 μm aresuccessively stacked on the n-GaAs substrate 1.

[0007] 2) A ridge mesa having a width of about 2 to 3 μm and length ofabout 800 μm is created on the epi-wafer using photolithographytechnique. As a result, length of the cavity of this semiconductor laserelement becomes 800 μm.

[0008] 3) The cap layer 9 from the anti-reflection side end facet, thatis, from the laser-emission side end facet F1 is to a width of 25 μm isremoved by selective etching to obtain the current non-injectionstructure. The reference numeral F2 denotes the laser-reflection sideend facet.

[0009] The cap layer may be removed using the semiconductor etchantdisclosed, for example, in Japanese Patent Application Laid-Open No.7-7004. This reference discloses an etchant that selectively etches onlythe GaAs layer when there exists layers of GaAs and AlGaAs. This methodis therefore called selective etching. The etchant is prepared by addinga basic compound to a mixture of organic acid and hydrogen peroxidebased mixture in such a manner that the pH of the mixture is between 6.0and 8.0. The organic acid is, for example, citric acid.

[0010] Precisely, aqueous citric acid solution (1% by weight) andaqueous solution of hydrogen peroxide solution (30% by weight) are mixedat a volume ratio of 100:1. Ammonia is added to this mixture in such amanner that the pH of the mixture is between 6.0 and 8.0. Assume thatthe ratio of the etching rate of the p-GaAs cap layer 9 to that of thep-Al_(0.35)Ga_(0.65)As upper clad layer 7 is called as selection ratio.Then, the p-GaAs cap layer 9 is etched more effectively when theselection ratio is high and etching can be stopped exactly at thep-Al_(0.35)Ga_(0.65)As upper clad layer 7. Based on an experiment it wasconfirmed that the selection ratio is 85 when the pH of the etchant is7.0. In an another experiment the p-GaAs cap layer 9 was removed usingthe etchant having the pH 7.0.

[0011] 4) Both the surfaces of the ridge mesa were then covered with theSiN film 10. Finally, p-electrode 11 and n-electrode 12 are stacked tohave the semiconductor laser element. This semiconductor laser elementis also called a ridge waveguide type semiconductor laser element.

[0012]FIG. 11 shows a cross of another conventional semiconductor laserelement. The difference between the semiconductor laser element shown inFIG. 11 and that shown in FIG. 9 is that the semiconductor laser elementshown in FIG. 11 has p-Al_(0.15)Ga_(0.85)As resistance control layer 8formed between the p-GaAs cap layer 9 and the p-Al_(0.35)Ga_(0.65)Asupper clad layer 7. It is known that the resistivity of the p-GaAs caplayer 9 can be reduced by employing such a structure. FIG. 12 shows across section of the semiconductor laser element along the line A-Ashown in FIG. 11. The current non-injection structure is formed byselectively etching a region from the laser-emission side end facet ofthe cap layer 9 and the resistance control layer 8. The width of thisremoved region is 25 μm. In other words, in this semiconductor laserelement it is necessary to etch both the p-GaAs cap layer 9 and thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 in one etching.

[0013] However, when both the p-GaAs cap layer 9 and thep-Al_(y)Ga_(1−y)As layer 8 are to be removed by one etching, dependingon the composition ratio y of aluminum, a small portion ofp-Al_(y)Ga_(1−y)As resistance control layer 8 is disadvantageouslyleftover as it is without being etched.

[0014] If even a small portion of the p-Al_(y)Ga_(1−y)As resistancecontrol layer 8 remains then the end facet corrosion occurs when thelaser output is increased. FIG. 11 and FIG. 12 show a portion 8 a, ofthe layer 8, that remained above the upper clad layer 7 because ofincomplete etching. When this portion 8 a remains above the upper cladlayer 7, which is a current non-injection layer, injection current flowsinto the laser-emission side end facet of the active layer 5 throughthis non-etched region 8 a. When current flows in the active layer 5,the above-mentioned cycle of positive feedback occurs, and opticaldamage and end facet corrosion occur at the laser-emission side endfacet.

[0015] Thus, with the conventional technology, it is possible toeffectively etch only the GaAs layer when there are layers of GaAs andAlGaAs. However, it is almost impossible to completely etch the p-GaAslayer and the p-Al_(y)Ga_(1−y)As layer in one etching when there arelayers of p-GaAs, p-Al_(y)Ga_(1−y)As, and p-Al_(x)Ga_(1−x)As.

SUMMARY OF THE INVENTION

[0016] It is an object of the present invention to provide asemiconductor laser element having an improved current non-injectionstructure. It is another aspect of this invention to provide an etchantwhich can selectively and completely etch desired layers in one etchingwithout harming the other layers. It is still another aspect of thisinvention to provide a method of fabricating the semiconductor laserelement according to the present invention using the etchant accordingto the present invention.

[0017] According to the semiconductor laser element of one aspect ofthis invention, desired layers are completely etched without harmingother layers. Furthermore, an insulating layer that has substantiallythe same thickness as the thickness of the etched layers is formed inthe portion from where the layers are etched. The layers may be etchedonly near the laser-emission side end facet, or may be etched near boththe laser-emission side end facet and the laser-reflection side endfacet.

[0018] According to the semiconductor laser element of another aspect ofthis invention, desired layers are completely etched without harmingother layers. Furthermore, an insulating layer that covers only thesurfaces of the layers that were exposed due to the etching is formed.

[0019] According to the etchant of still another aspect of thisinvention, when there are first, second, and third layers from top tobottom in order in a semiconductor laser element, then an etchant forwhich the ratio of dissolution rates of the first semiconductor layer tothe third semiconductor layer is between 10 and 20 is used.

[0020] Other objects and features of this invention will become apparentfrom the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 shows a cross section of a semiconductor laser elementaccording to a first embodiment of this invention.

[0022]FIG. 2 shows a cross section of the semiconductor laser elementshown in FIG. 1 along the line A-A.

[0023]FIG. 3A to FIG. 3D show how the semiconductor laser element shownin FIG. 1 is fabricated.

[0024]FIG. 4 is a graph that shows a relation between the pH of etchantand the selection ratio.

[0025]FIG. 5 shows a cross section of a semiconductor laser elementaccording to a second embodiment of this invention.

[0026]FIG. 6 is a table that shows a comparison between thesemiconductor laser elements formed using the etchant according to thefirst and second embodiments and the semiconductor laser element formedusing the conventional etchant.

[0027]FIG. 7 shows a cross section of a semiconductor laser elementaccording to a third embodiment.

[0028]FIG. 8A to FIG. 8D show how the semiconductor laser element shownin FIG. 7 is fabricated.

[0029]FIG. 9 shows a cross section of a conventional semiconductor laserelement.

[0030]FIG. 10 shows a cross section of the semiconductor laser elementshown in FIG. 9 along the line A-A.

[0031]FIG. 11 shows a cross section of another conventionalsemiconductor laser element which has a resistance control layer.

[0032]FIG. 12 shows a cross section of the semiconductor laser elementshown in FIG. 11 along the line A-A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Three embodiments of the present invention will be explained indetail below with reference to the accompanying drawings. However, thisinvention should by no means limited to these embodiments.

[0034] A cross of the semiconductor laser element according to a firstembodiment of this invention is shown in FIG. 1. A cross section of thesemiconductor laser element shown in FIG. 1 along the line A-A is shownin FIG. 2. The semiconductor laser element according to the firstembodiment is fabricated in the manner as shown in FIG. 3A to FIG. 3D.

[0035] The semiconductor laser element according to the first embodimenthas the same structure as that of the conventional semiconductor laserelement shown in FIG. 11 and FIG. 12. The difference between the two isthat, current non-injection structure can surely be formed in thesemiconductor laser element according to the first embodiment. This hasbecome possible because of the use of an etchant according to thisinvention which completely etches the desired layers.

[0036] The semiconductor laser element according to the first embodimentis fabricated in the manner explained below. To begin with, as shown inFIG. 1 and FIG. 2, then-GaAs substrate 1 is formed. Then, the n-GaAs(n=1×10¹⁸ cm⁻³) buffer layer 2 of thickness 0.5 μm, n-AlGaAs (n=1×10¹⁸cm⁻³) lower clad layer 3 of thickness 1.5 μm, n-GaAs (n=3×10¹⁷ cm⁻³)lower optical confinement layer 4 of thickness 0.03 μm,p-In_(0.2)Ga_(0.8)As (p=3×10¹⁷ cm⁻³) active layer 5 of thickness 80 Å,p-GaAs (p=3×10¹⁷ cm⁻³) upper optical confinement layer 6 of thickness0.03 μm, and p-Al_(0.35)Ga_(0.65)As(p=1×10¹⁸ cm⁻³) upper clad layer 7 ofthickness 1.2 μm are successively stacked above the n-GaAs substrate 1.

[0037] Subsequently, as shown in FIG. 3A in more detail, thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 of thickness 0.5 μmand the p-GaAs (p=4×10¹⁹ cm⁻³) cap layer 9 of thickness 0.03 μm aresuccessively stacked above the upper clad layer 7. Then, as shown inFIG. 3B, a portion of the cap layer 9 and resistance control layer 8beginning form the laser-emission side end facet F1 is completelyremoved using an etchant. The width of the etched portion is 25 μm. As aresult, top surface of the upper clad layer 7 in a portion E near thelaser-emission side end facet F1 is exposed. The reference symbol F2denotes the laser-reflection side end facet.

[0038] Thereafter, as shown in FIG. 3C, SiN film 10 is formed in theportion from where the cap layer 9 and the resistance control layer 8were removed. At this time, the upper clad layer 7, the resistancecontrol layer 8, and the cap layer 9 are etched into a mesa stripe, andthe SiN film 10 is also formed around the periphery of the etched area.Then, the p-electrode 11 and n-electrode 12 are formed to have the ridgewaveguide type semiconductor laser element. Finally, a coating of ananti-reflection film is created on the laser-emission side end facet F1,and a coating of optical reflection film is created on thelaser-reflection side end facet F2.

[0039] The etchant used for selectively etching the cap layer 9 andresistance control layer 8 is prepared as follows. Aqueous citric acidsolution (1% by weight) and aqueous hydrogen peroxide solution (30% byweight) are mixed in a volume ratio of 100:1. The pH of this mixture isadjusted to be 7.5 by adding ammonia to the mixture. The selectionration, i.e. the ratio of the etching rate of the p-GaAs layer to thatof the p-Al_(0.35)Ga_(0.65)As layer, is about 18 in this case. Theobtained etchant will be referred to as etchant T1. The etchant T1 isused to selectively etch only the p-GaAs cap layer 9 andp-Al_(0.15)Ga_(0.85)As resistance control layer 8 without harming thep-Al_(0.35)Ga_(0.65)As upper clad layer 7. The p-Al_(0.15)Ga_(0.85)Asresistance control layer 8 is completely removed in this etching.

[0040] The etchant is not limited to the one described above. Anotheretchant will be explained here. This etchant is obtained by mixingaqueous citric acid solution (1% by weight) and aqueous hydrogenperoxide solution (30% by weight) in a volume ratio of 100:1. The pH ofthis mixture is adjusted to be 6.0 by adding ammonia to the mixture. Theselection ratio in this case is about 10. The obtained etchant will bereferred to as etchant T2.

[0041] The relation between the pH of the etchant and the selectionratio will be explained below with reference to FIG. 4. After numerousexperiments, it was confirmed that the p-GaAs cap layer 9 and thep-Al_(y)Ga_(1−y)As resistance control layer 8 can be completely etchedand the p-Al_(x)Ga_(1−x)As upper clad layer 7 remains unharmed when theselection ratio is between 10 and 20. From FIG. 4 it can be seen that,the selection ratio will be between 10 and 20 when the pH of the etchantis between 6.0 to 6.1 or between 7.4 to 7.6. The selection ratio to pHcurve is steeper when the pH is between 6.0 and 6.1. The selection ratioto pH curve is gentle when the pH is between 7.4 and 7.6. This meansthat, stable and effective etching can be performed when the pH of theetchant is between 7.4 to 7.6. Therefore, the pH of the etchant shouldpreferably be between 7.4 and 7.6. However, better results will beobtained when the pH is between 7.4 and 7.8.

[0042] According to the first embodiment, the pH of citric acid basedetchant is set between 6.0 to 6.1 or between 7.4 to 7.6 and this etchantis used for the selective etching to obtain the semiconductor laserelement. As a result, only the p-GaAs cap layer 9 andp-Al_(0.15)Ga_(0.85)As resistance control layer 8 are effectively etchedwithout harming the p-Al_(0.35)Ga_(0.65)As upper clad layer 7. Theobtained semiconductor laser element has an advantage that the lightoutput is high, intensity of light is low near the end facet, and thereis the p-Al_(y)Ga_(1−y)As layer which reduces resistivity of the p-GaAslayer. Better results are achieved when the pH of the etchant is between7.4 to 7.6.

[0043] In the first embodiment, a case is described in which the currentnon-injection structure is formed only on the laser-emission side endfacet F1. However, the current non-injection structure may be formed onboth the laser-emission side end facet F1 and the laser-reflection sideend facet F2. When the current non-injection structure is formed on thelaser-reflection side end facet F2, optical damage or end facetcorrosion that may occur at the laser-reflection side end facet F2 canreliably be prevented. A case in which the current non-injectionstructure is formed on both the laser-emission side end facet F1 and thelaser-reflection side end facet F2 will be explained as a secondembodiment.

[0044]FIG. 5 shows a cross section of the semiconductor laser elementaccording to the second embodiment. SiO₂ film 20 as insulating film isprovided near the laser-emission side end facet F1. Similarly, SiO₂ film21 as insulating film is provided near the laser-reflection side endfacet F2. The insulating SiO₂ films 20 and 21 correspond to the SiN filmin the first embodiment. In other words, current non-injection structureis provided on both the laser-emission side end facet F1 and thelaser-reflection side end facet F2. The depth of the currentnon-injection structure from the respective end facet is 25 μm.

[0045] The etchant used for selectively etching the cap layer 9 andresistance control layer 8 is prepared as follows. Ammonia is mixed withaqueous citric acid solution (1% by weight) to adjust the pH to 7.4.Then this mixture and aqueous hydrogen peroxide solution (30% by weight)are mixed in a volume ratio of 50:1. The pH of the resultant mixture isadjusted to be 7.4. The selection ratio in this case is about 15. Theobtained etchant will be referred to as etchant T3.

[0046] The etchant T3 is used to selectively etch only the p-GaAs caplayer 9 and p-Al_(0.15)Ga_(0.85)As resistance control layer 8 withoutharming the p-Al_(0.35)Ga_(0.65)As upper clad layer 7. Thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 was completely removedin this etching.

[0047] Then, the p-electrode 11 and n-electrode 12 are formed in themanner as already explained in the first embodiment. Finally, a coatingof an anti-reflection film is created on the laser-emission side endfacet F1, and a coating of optical reflection film is created on thelaser-reflection side end facet F2 in the manner as already explained inthe first embodiment.

[0048] The etchant is not limited to the one described above. Anotheretchant will be explained here. Ammonia is mixed with aqueous citricacid solution (1% by weight) to adjust the pH to 6.1. Then this mixtureand aqueous hydrogen peroxide solution (30%by weight) are mixed in avolume ratio of 50:1. The pH of the resultant mixture is adjusted to be6.1. The selection ratio in this case is about 15. The obtained etchantwill be referred to as etchant T4.

[0049] The result of comparison between the etchants T1 and T2 used inthe first embodiment, etchants T3 and T4 used in the second embodiment,and the conventional etchant will be explained below with reference toFIG. 6. Just for reference, the pH of the conventional etchant is 7.0and the selection ratio is 85.

[0050] Semiconductor laser elements were formed using the etchants T1,T2, T3, T4, and conventional etchant. Horizontal size of near-fieldpattern (NFP), and the rise ratio (ΔI_(op)) of threshold current afterthe reliability test at 100 mW, 50° C. for 1000 hours of thesesemiconductor laser elements were measured. The results are shown inFIG. 6.

[0051] The NFP size of the semiconductor laser elements obtained byusing the etchants T1, T2, T3, T4 according to the present invention isbroader than that of the semiconductor laser elements obtained by usingthe conventional etchant. Moreover, the rise ratio ΔI_(op) of thethreshold current is less in the semiconductor laser elements accordingto the present invention than that in the conventional semiconductorlaser element. This means that the light intensity near thelaser-emission side end facet F1 of the semiconductor laser elementsaccording to the present invention is less than that of the conventionalsemiconductor laser element. In other words, there will be lessdeterioration of the end facet of the semiconductor laser elementsaccording to the present invention than that of the conventionalsemiconductor laser element.

[0052] In the first and second embodiments a case is explained in whichan insulating film is filled in the portion from where the layers wereremoved by etching. However, the portion from where the layers wereremoved by etching may be covered (coated) with an insulating. This casewill be described below as a third embodiment.

[0053] Cross section of the semiconductor laser element according to thethird embodiment is shown in FIG. 7. The number and composition of thelayers below the layer 7 shown in FIG. 7 is the same as those shown inFIG. 1 or FIG. 5, therefore, their explanation will be omitted to avoidsimple repetition of explanation.

[0054] The differences between the semiconductor laser element shown inFIG. 7 and that shown in FIG. 5 are as follows. Insulating SiN films 32a and 32 b are provided instead of the insulating SiO₂ films 20 and 21.The SiN films 32 a and 32 b cover the surface of the lower clad layer 7that is exposed because of etching. Furthermore, the p-electrode 11covers the SiN films 32 a and 32 b and the cap layer 9.

[0055] Thus, current non-injection structure is provided on both thelaser-emission side end facet F1 and the laser-reflection side end facetF2. The depth of the current non-injection structure from the respectiveend facet is 25 μm.

[0056] The semiconductor laser element according to the third embodimentis fabricated in the manner as shown in FIG. 8A to FIG. 8D. The layersfrom the n-GaAs substrate 1 to the p-Al_(0.35)Ga_(0.65)As upper cladlayer 7 are stacked in the same manner as explained in the firstembodiment. Subsequently, as shown in FIG. 8A, p-Al_(0.15)Ga_(0.85)Asresistance control layer 8 of thickness 0.5 μm and the p-GaAs (p=4×10¹⁹cm⁻³) cap layer 9 of thickness 0.03 μm are successively stacked on theupper clad layer 7.

[0057] Moreover, as shown in FIG. 8B, portions of the cap layer 9 andresistance control layer 8 beginning form the laser-emission side endfacet F1 and the laser-reflection side end facet F2 are completelyremoved using an etchant. The width of the removed portion from each endfacet is 25 μm. As a result, portions shown by reference numerals E1 andE2 of the upper clad layer 7 are exposed through removal of theresistance control layer 8 near the laser-emission side end facet F1 andthe laser-reflection side end facet F2 are exposed. Thereafter, as shownin FIG. 8C, the insulating SiN films 32 a and 32 b of thickness 120 nmare deposited in such a manner that these insulating films cover theexposed portions of the upper clad layer 7, the sides of the cap layer 9and resistance control layer 8, and a small portion of the top surfaceof the cap layer 9. At this time, the upper clad layer 7, the resistancecontrol layer 8, and the cap layer 9 are etched into a mesa stripe, andthe SiN films 32 a and 32 b are also formed around the periphery of theetched area. The thickness of the insulating SiN films 32 a and 32 b isnot limited only to 120 nm. The thickness should preferably be 100 nm ormore. If the thickness is less than 100 nm then IL characteristicsdegrade.

[0058] Then, as shown in FIG. 8D, the p-electrode 11 is formed in themanner as already explained in the first embodiment. Similarly, althoughnot shown in this figure, the n-electrode 12 is also formed in themanner as already explained in the first embodiment. Finally, a coatingof an anti-reflection film is created on the laser-emission side endfacet F1, and a coating of optical reflection film is created on thelaser-reflection side end facet F2 in the manner as already explained inthe first embodiment.

[0059] The etchant T1 explained in the first embodiment is used forselectively etching the cap layer 9 and resistance control layer 8. Thep-Al_(0.15)Ga_(0.85)As resistance control layer 8 was completely removedin this etching.

[0060] According to the third embodiment, the current non-injectionstructure is formed by forming insulating SiN films 32 a and 32 b ofthickness 120 nm that cover the layers exposed in the selective etching.As a result, the current non-injection structures can be surely formedwith simple method.

[0061] The third embodiment explained a case in which the cap layer 9and resistance control layer 8 in the vicinity of both thelaser-emission side end facet F1 and the laser-reflection side end facetF2 were etched and insulating SiN films were formed in the vicinity ofboth the laser-emission side end facet F1 and the laser-reflection sideend facet F2. However, the cap layer 9 and resistance control layer 8near only the laser-emission side end facet F1 may be etched and aninsulating SiN film may be formed near only the laser-emission side endfacet F1.

[0062] Moreover, the third embodiment explained a case in which theinsulating films that cover the layers exposed in the selective etchingis made of SiN. However, the insulating films may be made of SiO₂ orAl₂O_(3.)

[0063] Furthermore, the third embodiment explained a case in which theetchant T1 is used to selectively etch the cap layer 9 and resistancecontrol layer 8. However, the etchants T2, T2, and T4 may be used.

[0064] Furthermore, it is mentioned in the first to third embodimentsthat the semiconductor laser element has a ridge mesa. However, thesemiconductor laser element need not be limited to the one having aridge mesa. When there exist a lamination comprising in order thep-Al_(x)Ga_(1−x)As, p-Al_(y)Ga_(1−y)As (where x>y>0.2) and p-GaAslayers, then the present invention can be applied to selectively etchthe p-Al_(y)Ga_(1−y)As and p-GaAs layers.

[0065] Furthermore, it is mentioned in the first to third embodimentsthat the organic acid is citric acid. However, the organic acid could beany organic acid such as malic acid, malonic acid, oxalic acid, tartaricacid.

[0066] Furthermore, it is mentioned in the first to third embodimentsthat the width of the etched portion from the end facet is 25 μm.However, this width may be 10 μm or more. When the width is less than 10μm the current non-injection region does not give proper effect.

[0067] Furthermore, it is confirmed with experiments that the curveshown in FIG. 4 is obtained when the volume ratio of the aqueous organicacid solution to aqueous hydrogen peroxide solution is between 1:1 and200:1. Thus, although specific figure of the volume ratio of these twosolutions have been mentioned in the first to third embodiments, thevolume ration could be any value between 1:1 and 200:1.

[0068] As explained above, according to the semiconductor laser elementof one aspect of this invention, desired layers can be completely etchedwithout harming other layers. Furthermore, an insulating layer which hassubstantially the same thickness as the thickness of the etched layersis formed in the portion from where the layers are etched. As a result,the current non-injection structure can be formed, and it becomespossible to decrease the intensity of light only near side end facet.The layers may be etched only near the laser-emission side end facet, ormaybe etched in the vicinity of both the laser-emission side end facetand the laser-reflection side end facet.

[0069] According to the semiconductor laser element of another aspect ofthis invention, desired layers can be completely etched without harmingother layers. Furthermore, an insulating layer which covers only thesurfaces of the layers that were exposed due to the etching is formed.As a result, the current non-injection structure can be formed, and itbecomes possible to decrease the intensity of light only near side endfacet. The layers may be etched only near the laser-emission side endfacet, or may be etched near both the laser-emission side end facet andthe laser-reflection side end facet.

[0070] According to the etchant of still another aspect of thisinvention, the ratio of dissolution rate of the first semiconductorlayer to the third semiconductor layer is between 10 and 20. As aresult, the second semiconductor layer can be removed completely.

[0071] Although the invention has been described with respect to aspecific embodiment for a complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A semiconductor laser element having alaser-emission side end facet, comprising: a semiconductor substrate; aclad layer stacked directly above said semiconductor substrate or abovesome other layer stacked above said semiconductor substrate; aresistance control layer stacked above said clad layer and whichcontrols a junction resistance between said clad layer and a cap layerstacked above said resistance control layer; said cap layer has a shapesuch that it narrows an electric flowing in active layers; wherein aportion of said cap layer and said resistance control layer in thevicinity of said laser-emission side end facet is removed by selectiveetching, and an insulating layer having a thickness which is the same asthe total thickness of said cap layer and said resistance control layeris formed in the portion that became hollow due to etching; and a topelectrode layer stacked above said cap layer and said insulating layer.2. The semiconductor laser element according to claim 1 , wherein saidsemiconductor laser element having a laser-reflection side end facet,and a portion of said cap layer and said resistance control layer in thevicinity of said laser-reflection side end facet is removed by selectiveetching, and an insulating layer having a thickness which is the same asthe total thickness of said cap layer and said resistance control layeris formed in the portion that became hollow due to etching.
 3. Thesemiconductor laser element according to claim 1 , wherein saidinsulating layer is made of SiN, SiO₂, or Al₂O₃.
 4. The semiconductorlaser element according to claim 1 , wherein the width from saidlaser-reflection side end facet of the etched portion of said cap layerand said resistance control layer is 10 μm or more.
 5. The semiconductorlaser element according to claim 1 , further comprising a bottomelectrode layer stacked below said semiconductor substrate.
 6. Asemiconductor laser element having a laser-emission side end facet,comprising: a semiconductor substrate; a clad layer stacked directlyabove said semiconductor substrate or above some other layer stackedabove said semiconductor substrate; a resistance control layer stackedabove said clad layer and which controls a junction resistance betweensaid clad layer and a cap layer stacked above said resistance controllayer; said cap layer has a shape such that it narrows an electricflowing in active layers; wherein a portion of said cap layer and saidresistance control layer in the vicinity of said laser-emission side endfacet is removed by selective etching, and an insulating film whichcovers the surface of said clad layer exposed due to the etching, sidefaces of said resistance control layer and said cap layer, and a portionof top surface of said cap layer; and a top electrode layer stackedabove said cap layer and said insulating film.
 7. The semiconductorlaser element according to claim 6 , wherein said semiconductor laserelement having a laser-reflection side end facet, and a portion of saidcap layer and said resistance control layer in the vicinity of saidlaser-reflection side end facet is removed by selective etching, and aninsulating film which covers the surface of said clad layer exposed dueto the etching, side faces of said resistance control layer and said caplayer, and a portion of top surface of said cap layer.
 8. Thesemiconductor laser element according to claim 6 , wherein saidinsulating film is made of SiN, SiO₂, or Al₂O₃.
 9. The semiconductorlaser element according to claim 6 , wherein the width from saidlaser-reflection side end facet of the etched portion of said cap layerand said resistance control layer is 10 μm or more.
 10. Thesemiconductor laser element according to claim 6 , further comprising abottom electrode layer stacked below said semiconductor substrate. 11.The semiconductor laser element according to claim 6 , wherein thicknessof said insulating film is 100 nm or more.
 12. An etchant forselectively etching only a first and second layers of a multilayeredsemiconductor laser element comprising at least a third layer and saidsecond and first layers stacked successively on a semiconductorsubstrate, wherein said second layer controls resistance of saidmultilayered semiconductor laser element, wherein said etchant has sucha composition that the ratio of dissolution rate of said firstsemiconductor layer to said third semiconductor layer is between 10 and20.
 13. The etchant according to claim 12 , wherein said first layer ismade of p-GaAs, said second layer is made of p-Al_(y)Ga_(1−y)As, saidthird layer is made of p-Al_(x)Ga_(1−x)As where x>y>0.2.
 14. Theetchantaccordingtoclaim12, wherein said etchant is a mixture of organicacid and hydrogen peroxide based mixture.
 15. The etchant according toclaim 12 , wherein the pH of said etchant is between 7.4 and 7.8.
 16. Amethod of fabricating a semiconductor laser element, said semiconductorlaser element having a laser-emission side end facet, the methodcomprising the steps of: stacking a semiconductor substrate; stacking aclad layer directly above said semiconductor substrate or above someother layer stacked above said semiconductor substrate; stacking aresistance control layer above said clad layer which controls a junctionresistance between said clad layer and a cap layer stacked above saidresistance control layer, said cap layer has a shape such that itnarrows an electric flowing in active layers; selective etching aportion of said cap layer and said resistance control layer in thevicinity of said laser-emission side end facet; stacking an insulatinglayer having a thickness which is the same as the total thickness ofsaid cap layer and said resistance control layer in the portion thatbecame hollow due to etching; and stacking a top electrode layer abovesaid cap layer and said insulating layer.
 17. The method according toclaim 16 , wherein said semiconductor laser element having alaser-reflection side end facet, the method further comprising the stepof: selective etching a portion of said cap layer and said resistancecontrol layer in the vicinity of said laser-reflection side end facet;and stacking an insulating layer having a thickness which is the same asthe total thickness of said cap layer and said resistance control layeris formed in the portion that became hollow due to etching.
 18. Themethod according to claim 16 , wherein the selective etching of said caplayer and said resistance control layer is carried using an etchant forwhich ratio of dissolution rates of said clad layer to said cap layer isbetween 10 and
 20. 19. The method according to claim 16 , wherein saidinsulating layer is made of SiN, SiO₂, or Al₂O₃.
 20. The methodaccording to claim 16 , wherein the width from said laser-reflectionside end facet of the etched portion of said cap layer and saidresistance control layer is 10 μm or more.
 21. The method according toclaim 16 , wherein said clad layer is made of p-GaAs, said resistancecontrol layer is made of p-Al_(y)Ga_(1−y)As, said cap layer is made ofp-Al_(x)Ga_(1−x)As where x>y>0.2.
 22. The method according to claim 16 ,wherein said etchant is a mixture of organic acid and hydrogen peroxidebased mixture.
 23. The method according to claim 17 , wherein the pH ofsaid etchant is between 7.4 and 7.8.
 24. The method according to claim17 , further comprising step of stacking a bottom electrode layer belowsaid semiconductor substrate.
 25. A method of fabricating asemiconductor laser element, said semiconductor laser element having alaser-emission side end facet, the method comprising the steps of:stacking a semiconductor substrate; stacking a clad layer directly abovesaid semiconductor substrate or above some other layer stacked abovesaid semiconductor substrate; stacking a resistance control layer abovesaid clad layer which controls a junction resistance between said cladlayer and a cap layer stacked above said resistance control layer, saidcap layer has a shape such that it narrows an electric flowing in activelayers; selective etching a portion of said cap layer and saidresistance control layer in the vicinity of said laser-emission side endfacet; stacking an insulating film which covers the surface of said cladlayer exposed due to the etching, side faces of said resistance controllayer and said cap layer, and a portion of top surface of said caplayer; and stacking a top electrode layer above said cap layer and saidinsulating film.
 26. The method according to claim 25 , wherein saidsemiconductor laser element having laser-reflection side end facet, themethod further comprising the step of: selective etching a portion ofsaid cap layer and said resistance control layer in the vicinity of saidlaser-reflection side end facet; and stacking an insulating film havinga thickness which is the same as the total thickness of said cap layerand said resistance control layer is formed in the portion that becamehollow due to etching.
 27. The method according to claim 25 , whereinthe selective etching of said cap layer and said resistance controllayer is carried using an etchant for which ratio of dissolution ratesof said clad layer to said cap layer is between 10 and
 20. 28. Themethod according to claim 25 , wherein said insulating film is made ofSiN, SiO₂, or Al₂O₃.
 29. The method according to claim 25 , wherein thewidth from said laser-reflection side end facet of the etched portion ofsaid cap layer and said resistance control layer is 10 μm or more. 30.The method according to claim 25 , wherein said clad layer is made ofp-GaAs, said resistance control layer is made of p-Al_(y)Ga_(1−y)As,said cap layer is made of p-Al_(x)Ga_(1−x)As where x>y>0.2.
 31. Themethod according to claim 25 , wherein said etchant is a mixture oforganic acid and hydrogen peroxide based mixture.
 32. The methodaccording to claim 25 , wherein the pH of said etchant is between 7.4and 7.8.
 33. The method according to claim 25 , further comprising stepof stacking a bottom electrode layer below said semiconductor substrate.34. The method according to claim 25 , wherein thickness of saidinsulating film is 100 nm or more.